Future active array antennas for platforms such unmanned airborne vehicles (UAVs) will require increased reconfigurabity to enhance performance, wide tunable frequency bandwidth and signature. In many applications, a groundplane needs to be placed behind the radiators of such antennas to shield any back side electronics and to enhance RF antenna performance. For optimum performance the distance between the groundplane and radiators should be kept to an electrical distance of a quarter wavelength. The problem is that the physical dimension for a quarter wavelength is fixed for a given frequency, thus the electrical distance will vary as the frequency changes across a wide band. The result is performance degradation of the antenna aperture as the electrical distance changes between the groundplane and wide band radiators.
FIG. 17 shows an illustration of a convention active phase array antenna. Typical installation on a platform requires that a groundplane be placed behind the radiators to provide RF shielding for the electronics and transmission lines located behind the aperture (i.e., TR module, phase shifters, manifolds, etc.). For a frequency bandwidth up to an octave, placement of the groundplane behind the radiator by one quarter of a wavelength at the center frequency provides optimum enhancement of the radiator performance.
Recently, wideband radiating element such as spirals, flare dipoles and long slots with greater than 5 to 1 frequency bandwidths are being used to realize ultra-wideband active arrays. As the frequency band increases, the quarter wavelength spacing between the radiator and the groundplane can no longer be maintained and the result is degradation of the radiator/array antenna performance due to interaction between the radiator and the groundplane.